Currently, most photolithographic apparatuses incorporate alignment systems based on the interference of grating diffracted beams. Such alignment systems have the basic features that the incidence of a single-wavelength or multi-wavelength illumination light beam on a phase grating alignment mark causes the diffraction of the light beam into sub-beams of different diffraction orders, each sub-beam containing an indication about the position of the alignment mark. The sub-beams with different diffraction orders scattered from the phase grating at different angles are collected by the alignment system, and those of the same diffraction order and with different diffraction order signs (e.g., ±1, ±2 and ±3 orders) are then mutually superimposed and interfere in pairs on an image plane or pupil plane of the alignment system, generating interference signals of the respective orders. As the alignment mark is scanned, a photoelectric detector records intensity variations in the interference signals and signal processing is further involved to determine the position of a center of the alignment mark.
The existing alignment systems are represented by an off-axis alignment system available from ASML Netherlands. This alignment system utilizes both red-light and green-light sources for supplying illumination beams, and an array of optical wedges or a group of wedge plates for accomplishing the superimposition and interferometric imaging of the sub-beams of different orders from the alignment mark and for partitioning the imaging space at the image plane. Alignment signals from the red and green light beams are split by a polarization splitting prism and an output sinusoidal signal is produced by detecting the light intensity of an image of the alignment mark passed by a reference grating. However, this alignment system has the following deficiencies: firstly, the polarization splitting prism based beam-splitting system can only handle beams with two wavelengths and is unable to handle alignment signals with more than two wavelengths; secondly, the interference of the sub-beams with various diffraction orders occurs at the image plane, which will lead to large alignment errors for an alignment mark having a non-uniform reflectivity upon the occurrence of several conditions, such as rotation of the alignment mark or presence of a magnification error; lastly, the manufacturing, assembly and adjustment of the wedge array are imposed with stringent requirements to ensure desired coincidence between inclined faces of the two wedge-shaped plates of each pair for diffracting positive and negative order sub-beams of the same diffraction order in terms of shape and slope. Such requirements will lead to high engineering difficulties and high costs.
Another known off-axis alignment system is also from ASML Netherlands. This system uses a rotating self-referencing interferometer to produce two images of the alignment marker. The two images are rotated by 180° with respect to each other. Overlapping and interference of the sub-beams with different diffraction orders is arranged to take place at the pupil plane to generate interference signals. Relative phase differences of the interference signals of different orders are subsequently detected to determine an alignment position signal. This known alignment system utilizes the self-referencing interferometer taking the form of a composite prism with multiple principal cross sections which allows only small dimensional tolerances and hence imposes strict requirements on the manufacturing, assembly and adjustment of the interferometer. In addition, the prism is made up of several components glued together and such gluing is a difficult process. Furthermore, rotating a sub-beam by 180° and overlapping it with a non-rotated sub-beam to make them interfere in a desired manner requires the use of a light beam with high spatial coherence as the illumination beam for generating the sub-beams, and the process itself is of a high difficulty.
Therefore, there is an urgent need in this art for an off-axis alignment system and an associated method with no special requirement on spatial coherence of the illumination light beam, immunity of alignment results from the influence of tilting and defocusing of the alignment mark, simple optical path design, no necessity of using complex optics, and ease of construction.